This invention relates to frame-to-frame comb filters for separating the luminance (Y) and chrominance (C) components of composite television signals and more particularly to means for correcting signal distortion due to the occurrence of motion between successive video frames in such a comb filter system.
Electronic signals which are periodic in nature may be processed advantageously by storing replicas of the signal which are separated in time by the repetition period and then combining the stored replicas to enhance information content of the signal. For example, conventional NTSC television broadcast systems (and most video record/playback systems) are arranged so that much of the brightness (luminance) information contained in an image is represented by signal frequencies which are concentrated about integral multiples of the horizontal line scanning frequency. Color (chrominance) information is encoded or inserted in a portion of the luminance signal spectrum around frequencies which lie halfway between the multiples of line scanning frequency (i.e., at odd multiples of one-half the line scanning frequency).
Chrominance and luminance information can be separated and detail information may be enhanced by appropriately combing the composite signal spectrum. Known combing arrangements take advantage of the fact that the odd multiple relationship between chrominance signal components and half the line scanning frequency causes the chrominance signal components for corresponding image areas on successive lines to be 180.degree. out of phase with each other (so-called interlaced frequency components). Luminance signal components for corresponding image areas on successive lines are substantially in phase with each other (and comprise non-interlaced components).
In a comb filter system, one or more replicas of the composite image-representative signal are produced which are time delayed from each other by at least one line scanning interval such that the phase of delayed and non-delayed chrominance component are 180.degree. out of phase. The delayed signals are added to non-delayed signals, resulting in the cancellation of the interlaced frequency components (e.g., chrominance) while reinforcing the non-interlaced frequency components (e.g., luminance). By subtracting the delayed and non-delayed signals (e.g., by inverting one of the signals and then adding the two), the non-interlaced frequency components are cancelled while the interlaced frequency components are reinforced. Thus, the luminance and chrominance signals may be mutually combed and thereby may be separated advantageously. Generally, comb filtered signal components contain lesser cross components than luminance and chrominance signal components that are separated from composite video signal by other previously employed techniques.
Interline comb filters, however, produce several undesirable features in reproduced images. First, there is a loss in vertical resolution resulting from the 15.734 KHz nulls in a 1H comb filter frequency response. (The vertical resolution can be restored, however, with appropriate additional circuitry.) Secondly, line comb filters produce "hanging dots" along sharp vertical image transitions. "Hanging dots" occur when signals from line-to-line are not substantially alike precluding the comb filter from cancelling the chrominance signal component at the luminance output of the comb filter. Thirdly, the interline comb filter produces an inherent loss of resolution along specific picture information diagonals. Because of these and other shortcomings, alternatives to the line comb filter have been sought. A solution, at least for stationary pictures, is a frame-to-frame comb filter. By design, the chrominance signal of an NTSC composite video signal is synchronized to have a 180 degree phase relationship from frame-to-frame permitting comb filtering on a frame basis. The luminance component may be extracted by linear addition of the composite signal from successive frames and the chrominance component may be extracted by linear subtraction of the composite signal from successive frames.
Consider a stationary picture, and a particular pixel of that scene. The signal redundancy or similarity for that pixel will be substantially one hundred percent from frame-to-frame. Thus if comb filtering is performed on a frame-to-frame basis, cancellation of the undesired video component in both the comb filtered luminance component and the comb filtered chrominance component will be complete regardless of image detail. "Hanging dots" do not occur because the frame comb filter combines luminance and chrominance information from the same picture point in contrast to line comb filters which combine signals from vertically adjacent picture points which may contain different information. The frame comb filter suffers neither a loss in resolution in the vertical direction nor along picture diagonals.
The frame comb filter does produce image distortions around objects in reproduced images which move between frame intervals. Differences in scene content due to object motion or camera panning that occur in the time frame of one-thirtieth of a second or faster are characterized as being visual motion of the scene content. In a frame comb filter, visual motion tends to produce image distortions around moving objects. The distortions are observable in both the horizontal and vertical directions in the plane of the image and are manifested as dual images in the reproduced scenes. The dual images are separated by an amount corresponding to the rate of motion and may consist of incorrect hue at the edges of moving colored objects, objectionable dot structure on the edges of colored objects, and low frequency color edge beats.
The image distortions result primarily from two sources. The first is an effective time lag of the luminance signal wherein edges of a moving object are represented by a signal average of the moving edge and the scene background. This effect tends to produce phantom images of the moving object. The second source of distortion results from motion induced introduction of luminance cross components into the comb filtered chrominance output signal and chrominance cross components in the comb filtered luminance output signal.
Phantom images may be considered a first order distortion and may mask distortions due to cross components. If the phantom image distortion is corrected, however, then the distortion due to luminance cross components in the chrominance signal become significant and is manifested by hue errors circumscribing moving objects, color edge beats, etc.
Phantom images may be removed by extracting the luminance cross components from the combed chrominance signal and inserting them back into the combed luminance signal. It is an object of this invention to cure the distortions due to luminance signal contaminating the combed chrominance signal.